Fix missing "!" in cube map render target serial check.

BUG=angle:849

Change-Id: I9e4cb31bdc82a1c556908571355387a9698c7528
Reviewed-on: https://chromium-review.googlesource.com/239230
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/TextureD3D.cpp b/src/libANGLE/renderer/d3d/TextureD3D.cpp
index 0634ab2..48101fa 100644
--- a/src/libANGLE/renderer/d3d/TextureD3D.cpp
+++ b/src/libANGLE/renderer/d3d/TextureD3D.cpp
@@ -1426,7 +1426,7 @@
 
 unsigned int TextureD3D_Cube::getRenderTargetSerial(const gl::ImageIndex &index)
 {
-    return (ensureRenderTarget().isError() ? mTexStorage->getRenderTargetSerial(index) : 0);
+    return (!ensureRenderTarget().isError() ? mTexStorage->getRenderTargetSerial(index) : 0);
 }
 
 gl::Error TextureD3D_Cube::getRenderTarget(const gl::ImageIndex &index, RenderTarget **outRT)